The present invention relates to the field of coated sharp edged cutting tools made of or comprising a sintered body embracing at least a hard material and a binder material which has been sintered under temperature and pressure to form the body.
With past and current sintering technology of powder metallurgy cemented carbide cutting tools have been used both in uncoated and in CVD and PVD coated conditions. CVD as well as MT-CVD coating processes need high temperatures, usually above 950° C. for HT-CVD or between 800° C. and 900° C. for MT-CVD, and a chemically aggressive process atmosphere. This has, amongst others, well known drawbacks with reference to transverse rupture strength (TRS) and low edge strength of the cutting tools as well as to unavoidable thermal cracks of the coating.
A closer look to the drawbacks of HT (high temperature)-CVD should be given in the following with the coating of cemented carbides taken as an example:    a) As mentioned, reduction of TRS of the substrate—may be due to the fact that the surface state prior to coating is one of residual compressive stress induced by the correct grinding process, which is beneficial; this state is altered by high temperature which relieves this beneficial residual compressive stress. Therefore, independent of the coating, high temperature annealing has this effect on the carbide substrate. However, even if the substrate is not properly ground—for instance, if it is subjected to “abusive grinding” which leaves residual tensile stress or even some surface cracks—the high temperature treatment has essentially no beneficial effect.    b) A further reduction of the TRS of the coated tool comes from the presence of thermal cracks induced by thermal expansion mismatch between the coating and substrate upon cooldown from the high CVD temperature. The cracks run through the thickness of the coating, and thus can initiate fatigue failure under certain cutting conditions.    c) In the case of WC—Co hardmetals, it is also known that cobalt diffuses towards the surface with temperatures of about 850° C. and above which is also associated with decarburization and eta phase formation during the CVD process. Such eta phase can e.g. be formed by the decarburization of the outer region of the substrate in the initial formation of TiC or TiCN CVD first layer which is the usual underlayer for CVD Al2O3 coating layer. The eta phase region forms an embrittled layer with high porosity, again causing micro-cracking initiation sites as well as coating delamination tendency. At least this drawback of HT-CVD has been overcome with MT (medium temperature)-CVD e.g. by applying a first TiCN layer at about 850° C., thereby minimizing substrate eta phase formation.
Therefore different measures have been taken to diminish such detrimental effects. U.S. Pat. No. 4,610,931 suggests using cemented carbide bodies having a binder enrichment near the peripheral surface. In U.S. Pat. No. 5,266,388 and U.S. Pat. No. 5,250,367 application of a CVD coating being in a state of residual tensile stress followed by a PVD coating being in a state of residual compressive stress has been suggested for as mentioned binder enriched tools.
Despite the fact that cemented carbides have been used to illustrate the drawbacks of CVD coating processes above the same or at least similar problems are known from other substrates having sintered bodies. Cermets also have Co, Ni (and other metals like Mo, Al, . . . ) binders and undergo a sintering process similar to cemented carbides. TiCN-based cermets e.g. are not as readily CVD-coated today as these substrates are more reactive with the coating gas species, causing an unwanted reaction layer at the interface. Superhard CBN tools use high-temperature high-pressure sintering techniques different from that used for carbides and cermets. However they may also have metallic binders such as Co, Ni, . . . tending to high temperature reactions during CVD coating processes. These substrates are sometimes PVD-coated with TiN, TiAlN, CrAlN or other coating systems mostly for wear indication at the cutting edges. Such coatings however can only give a limited protection against high temperature and high oxidative stress due to high cutting speeds applied with state of the art turning machines as example.
Ceramic tool materials based on solid Al2O3, Al2O3—TiC; or Al2O3—Si3N4 (SiAlON) that incorporate glassy phases as binders represent another tool type which are electrically insulating and therefore difficult to coat also with conventional PVD. These materials are sinter-HIPped, as opposed to lower-pressure sintered carbides. Such ceramic inserts again are not CVD coated because high temperature can cause softening of the Si3N4 substrate or cause it to lose some toughness as the amorphous glassy binder phase becomes crystalline. Uncoated materials however can allow interaction during metal cutting between their binder phases and the workpiece material and therefore are susceptible to cratering wear restricting use of such tools to limited niche applications.
Therefore PVD coatings have replaced CVD coatings in parts or even completely for many operations with high demands on tool toughness or special needs on geometry. Examples for such tools are tools used for interrupted cut applications like milling or particularly sharp-edged threading and tapping tools. However due to outstanding thermochemical resistivity and hot hardness, oxidic CVD-coatings as e.g. Al2O3 in α- and/or γ-crystal structure, or with needed thick multilayers comprising such coatings are still in widespread use especially for rough-medium turning, parting and grooving applications in all types of materials and nearly exclusively with turning of cast iron. Such coatings could not be produced by PVD processes until recently due to principal process restrictions with electrically insulating materials and especially with oxidic coatings.
As is well known to the person of ordinary skill in the art all the problems as mentioned above tend to occur and focus on the cutting edge becoming more acute with the smaller radius of the cutting edge. Therefore to avoid edge chipping or breaking with CVD coated tools additional geometrical limitations have to be considered for cutting edges and tool tips, with cutting edges limited to a minimum radius of 40 μm for cemented carbides for example. Additionally further measures like applying a chamfer, a waterfall, a wiper or any other special geometry to the clearance flank, the rake face or both faces of the cutting edge are commonly used but add another often complex-to-handle production step to manufacturing of sintered tool substrates.